偏振优化设计的大口径环形太阳望远镜光学系统

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	偏振优化设计的大口径环形太阳望远镜光学系统
其他题名	The optical system of large aperture ring Solar telescope with polarization optimization design
	付玉
学位类型	博士
导师	刘忠
	2022-07-01
学位授予单位	中国科学院大学
学位授予地点	北京
培养单位	中国科学院云南天文台
学位专业	天文技术与方法
关键词	环形太阳望远镜大口径光学系统仪器偏振优化设计
摘要	8米中国巨型太阳望远镜，简称CGST（Chinese Giant Solar Telescope ），是中国下一代地基大口径太阳望远镜建造计划。其核心目标是通过高精度、高分辨的偏振光谱观测，系统性的研究太阳光球到高色球中空间尺度20km目标的矢量磁场、速度场和热力学结构，为解决当前太阳物理和空间天气研究中面临的关键科学问题奠定基础。因此，CGST将是一台具有高空间分辨率和高偏振测量精度的望远镜。 CGST光学系统设计难点是在可见光及近红外波段实现衍射极限成像，以及在He I1.083 um和 Fe I 1.565 um磁敏谱线所在的近红外波段，CGST 仪器偏振小于 2×10−4，同时保证望远镜的仪器偏振不随时间变化而改变。本文针对以上问题进行深入研究，具体研究方法及成果如下：（1）地基太阳望远镜通常选择格里高利光学结构，论文计算了格里高利焦点在主镜前和在主镜后两种情况下光学系统的初始参数，再利用 Zemax 软件对光学系统进行优化设计，当光学系统综合焦比不同时，比较了光学系统的赛德尔像差系数和成像质量。结果表明，格里高利焦点在主镜之前或者主镜之后的设计，望远镜的成像质量都能满足科学目标的要求，即达到衍射极限成像，当格里高利焦点在主镜之后时，热光阑的安装空间更大。基于数值仿真方法，分析了主副镜材料与热效应的关系，为 CGST 热控方案提供有利参考。（2）CGST 的光学系统中光学元件都是反射镜，金属反射膜的偏振特性是本论文要讨论的重要一部分。基于金属薄膜理论，研究了入射角，波长，以及电介质保护膜厚度对金属薄膜反射偏振特性的影响。利用自主开发的偏振光线追迹程序，分析了典型的折轴镜仪器偏振的焦比和视场特性，并提出了一种新型四镜偏振补偿折轴方案。该方案具有偏振补偿效果对光束的入射焦比和视场角不敏感的优点，并能够克服两镜偏振补偿折轴方案光轴平移的缺点。（3）针对 CGST 的低仪器偏振设计需求，给出了四种望远镜折轴方案。利用偏振光线追迹方法，分析了这些方案仪器偏振的孔径综和视场效应，及这些效应随波长和望远镜高角度的变化特征。其中，由两镜和四镜偏振补偿机构共同组成的完全偏振补偿折轴方案，具有在任意波长和望远镜角度下中心视场仪器偏振为零的优点。该方案具有一定的视场效应，在近红外波段其最大无偏振视场为 1arcmin，在可见光波段该视场为 0.5 arcmin，完全符合 CGST 低仪器偏振的设计要求。其它方案也各自具有不同的特点和优势，可以作为 CGST 偏振光学设计的备选方案。（4）CGST 将通过主动光学系统与自适应光学系统的协同工作消除重力弯沉、结构热变形等因素对望远镜像质的影响。精确的主动对准是 CGST 主动控制的重要任务。为满足可见光波段近衍射极限的像质要求，本论文提出了基于激光干涉测距的激光桁架主动对准方法。当激光桁架的测距不确定性要在 5𝜇m 以内，环境温度梯度小于 0.381K/m 时，能到达主副镜对准要求。并且在 Kolmogorov 湍流和典型昼间近地面湍流条件下，大气湍流并不会严重降低激光桁架测量的灵敏度。从理论上论证了激光桁架在大口径太阳望远镜主副镜光轴对准中使用的可行性。
其他摘要	The 8-meter China Giant Solar Telescope, or CGST (Chinese Giant Solar Telescope), is the next generation of ground-based large-aperture Solar telescopes in China. Its core aims are to study the vector magnetic field, velocity field, and thermodynamic structure of 20km spatial targets,from the solar sphere to the high chromosphere. The high-precision and high-resolution polarization spectroscopic observations will lay a foundation for solving the scientific problems in the current solar physics and space weather research. Therefore, CGST will be a telescope with high spatial resolution and high polarization measure accuracy.The difficulty in the design of the CGST optical system is to realize diffraction limit imaging in visible and near-infrared wavelengths, and the instrument polarization of CGST is less than 2×10−4 in near-infrared wavelengths, He I1.083 μ m and Fe I 1.565 μ m magnetic sensitive spectral lines are located. Meanwhile, the instrument polarization of the telescope does not change with time. This paper conducts in-depth research on the above issues. The specific research methods and results are as follows:(1)Usually, the Gregory optical structure is considered a good choice for the grand-based solar telescope. In this paper, we calculated the initial parameters of the two different optical systems, with the Gregory focus located after the primary mirror and before the primary mirror. And then, using the Zemax software to optimize the optical system design, compared the focal rate of Seidel aberration and the imaging quality with two structures. The results show that the imaging quality of the telescope can meet the scientific goal of diffraction limit imaging whenever the focus is before or after the primary mirror. However, when the Gregory focus is located after the primary mirror, the room for the heat-stop is big enough. Based on the numerical simulation methods, the relationship between the optical element materials and the thermal effect is presented, which provides references for the CGST thermal control scheme.(2)The optical elements in CGST optical system are all mirrors, and the polarization characteristics of metal reflection film are also a vital part of this paper. Based on the theory of polarization characteristics of plane electromagnetic wave, the negative refractive index of the metal film, the reflection of the metal surface, and the structure of multilayer complex reflective film are analyzed, and the polarization characteristics of the metal film are summarized. The polarization characteristics of a typical folding mirror, two mirror compensation systems, and four mirror compensation systems are analyzed using polarization ray-tracing software. The results show that the instrument polarization introduced by the four-mirror compensation structure is less than the two mirror compensation and folding mirrors. Aiming at the aging problem of the film by sunlight, we designed the experiment for contrast. Taking aluminum film as an example, we compared the change of the polarization characteristics of the film before and after exposures under the sunlight，the results indicated that the instrument polarization increased with the increase of sunlight duration.(3)Considering the CGST should have the characteristics of low instrument polarization, this paper proposes a telescope folding optical system design scheme based on a four-mirror polarization compensation structure and two-mirror mirror compensation structure. The instrument polarization in the different fields of view, at the different wavelengths, and introduced with the change of the telescope's elevating angles are analyzed by the methods of polarization ray tracing. The results show that the four-mirror folding optical system is superior to the two-mirror folding structure. With the four-mirror folding optical system, the “non-polarized field of view” of the CGST instrument, whose polarization meets the measurement accuracy requirement of 2×10−4, is 1 arc- minute at the near-infrared band, and at the visible waveband, the “non-polarized field of view” is 0.5 arc-seconds.(4)To assure the image quality, eliminate the influence of gravity deflection and structural thermal deformation is necessary. Accurate active alignment is an important task of CGST active control. To meet the image quality requirement of near diffraction limit of visible band, the wave-front RMS caused by the alignment error of primary and secondary mirrors should not exceed 1/10 wavelength. In this paper, an active alignment method of laser truss based on laser interferometry is proposed. Motion analysis and optical modeling show that the laser truss can meet the alignment requirements of primary and secondary mirrors when the ranging uncertainty is less than 5 microns and the ambient temperature gradient is less than 0.381K/ m in the visible band. In addition, atmospheric turbulence does not significantly reduce the sensitivity of laser truss measurements under Kolmogorov turbulence and typical diurnal near-surface turbulence. Therefore, the laser truss measurement method can meet the requirements of CGST secondary mirror active alignment. In this paper, the feasibility of laser truss is proved theoretically, which lays a foundation for the implementation of this technology and the future engineering application.
学科领域	物理学 ; 光学 ; 天文学 ; 太阳与太阳系 ; 太阳与太阳系其他学科 ; 机械工程 ; 仪器仪表技术
学科门类	理学 ; 理学::天文学 ; 工学 ; 工学::光学工程 ; 工学::仪器科学与技术
页数	0
语种	中文
文献类型	学位论文
条目标识符	http://ir.ynao.ac.cn/handle/114a53/25793
专题	天文技术实验室
推荐引用方式 GB/T 7714	付玉. 偏振优化设计的大口径环形太阳望远镜光学系统[D]. 北京. 中国科学院大学,2022.

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